The performance of three-phase or other multiphase electrical apparatus, motors in particular, is adversely affected when there is an unbalance between the voltages supplied by the respective phases of the multiphase system, even though the average of the phase-voltage is well within the acceptable range of supply voltage for the equipment. Of course, abnormally high-voltage or abnormally low-voltage on all phases is also potentially detrimental to electrical equipment. In particular, phase unbalance is detrimental when it persists for long periods of time because it can lead to overheating of a particular phase winding of a motor or the like resulting in a cumulative deterioration and shortened life or even catastrophic failure. Apparatus for monitoring multiphase electrical systems to detect phase unbalance or high or low-voltage conditions and to provide false signals to operate protective relays or visual or audible indicators have long been well known. Typically such monitor apparatus has been bulky with numerous expensive components or failed to achieve optimum reliability and avoid shutdown or false warning from relatively innocuous transient conditions.